During certain semiconductor assembly processes, semiconductor dice are placed on a carrier such as a leadframe substrate. Electrical connections in the form of wire bonds are then made between the dice and substrate, or between individual dice. Gold, aluminium or copper wires are commonly used to make these connections. Wire bonds are formed at bonding sites where the electrical connections are to be made, typically using an ultrasonic transducer to generate ultrasonic energy to attach a length of wire fed from a capillary to the semiconductor device or carrier. After these wire bonds are made, the dice, wire loops and substrate are encapsulated with a resin material to protect the same. A semiconductor package is thus produced.
There is a continuing desire in the semiconductor industry to develop ever smaller and thinner semiconductor packages. Since, as explained above, the wire loops should be fully encapsulated in the final package, the thickness of the package would be affected by the heights of the wire loops that are formed during wire bonding. If the heights of the wire loops can be kept to a minimum, then the thickness of the final package can be correspondingly reduced.
Furthermore, there is a demand in the industry for semiconductor devices with stacked dice. The advantage of having stacked dice is that stacked dice incorporate more silicon functionality by stacking multiple dice into a single package. This reduces overall size by eliminating additional packages on the circuit board. Furthermore, it increases space savings while enhancing electrical performance by reducing propagation time for signals to traverse from one chip to another. Stacked dice allow a greater density of integrated circuits on a given area of the carrier, and may increase efficiency. Since each die in the stack would require an electrical connection to the carrier, or to another die, several layers of wire loops are formed. Correspondingly, it would be better for wire loops to be profiled as low as possible to cater for this need.
Thus, there is a desire in the semiconductor industry to seek to address this need to form wire loops with low height profiles. For example, U.S. Pat. No. 6,933,608 entitled “Wire Loop, Semiconductor Device Having Same, Wire Bonding Method and Wire Bonding Apparatus” seeks to form a low wire loop by crushing a part of a bonding wire onto a top of a bonded ball with a capillary. The wire loop is then extended from the crushed wire.
A problem with this approach of crushing the bonding wire by bonding it to the ball bond is that the strength of the neck portion of the wire bond is weak. This makes it more susceptible to neck crack and breakage. As such, the wire pull strength of the wire bond is reduced. Further, the crushing of the bonding wire onto the ball bond may weaken the strength of the ball bond, resulting in weakened adhesion between the ball bond and the bonding surface. Moreover, additional wire is required to loop the wire backwards and then bond the wire to the ball. This wire that is looped backwards is unused. Thus, there is unnecessary wastage of bonding wire.